Cranston was a professor at London School of Economics from 1992 to 1997 and the holder of the Cassell chair in commercial law from 1993 to 1997. Before that he held academic posts in the UK and Australia and the Sir John Lubbock chair in banking law at QMW, being a professor of Law at Queen Mary and Westfield College from 1986 to 1991.

Cranston was the Centennial Professor of Law at the LSE from 2005 to 2007, and returned as a professor of law from 2017.[2]

Appointed as a High Court judge in October 2007, he was assigned to the Queen's Bench Division.[3]Marcel Berlins wrote in The Guardian at the time that Cranston's appointment was unusual among judicial appointments in recent years, given that it occurred so soon after the end of his political career.[4] Cranston retired with effect from 16 March 2017.[5]